1. Field of the Invention
Preferential complexation of one organic compound in a mixture of related compounds is a known technique for resolving mixtures of compounds having close boiling points. Of particular interest herein are methods for resolving mixtures of phenolic compounds, at least one of which is a chlorinated phenolic, by preferential complexation of one of the phenolics.
2. State of the Art
Mixtures of chlorinated phenolics are available as end-products or by-products of many commercial chlorination reactions involving the ring chlorination of phenol and cresols. Such reactions produce many closely-boiling chlorinated phenolics. The isolation and purification of individual chlorinated phenolics is very difficult by the use of conventional separation methods such as fractional distillation.
There are chemical processes known for separating closely-boiling organic compounds by methods other than, or in addition to, energy-intensive physical separation techniques such as fractional distillation or fractional crystallization. These chemical processes involve a step of preferential complexation of one component of a mixture of closely-boiling compounds over other components of the mixture. For example, U.S. Pat. No. 4,267,389 to Leston, describes treating a phenolic mixture comprising para-cresol, methylated phenols and ethylated phenols, with an inorganic halide salt, such as calcium bromide, to remove para-cresol from the mixture. Removal of para-cresol from the mixture involves formation of a complex between para-cresol and calcium bromide, which complex forms preferentially over complexes between calcium bromide and other components of the phenolic mixture.
Mixtures of various alcohols may be resolved by treatment with a halide salt. For example, in Sharpless et al., J. Org. Chem., Vol. 40, No. 9, p.p. 1252-1257 (1975), there is reported a study of competition between pairs of mono-hydroxy alcohols and mono-hydroxy phenols for complex formation with a halide salt. This study finds that phenols as a class form poorer complexes than alcohols of comparable melting point, probably because the phenols are weaker bases than the comparable alcohols.
There remains need, therefore, for methods for resolution of mixtures of closely-boiling chlorinated phenolics by chemical complexation methods, rather than by fractional crystallization or distillation.